1. Field of the Invention
The present invention relates to a nitride semiconductor light emitting device including a nitride semiconductor substrate and a light emitting layered structure that includes a plurality of nitride semiconductor layers stacked on the nitride semiconductor substrate.
2. Description of the Background Art
Conventionally, the characteristics of nitride semiconductors are utilized and studied for light emitting devices and high power devices. For instance, when a light emitting device is produced using a nitride semiconductor, the composition of the semiconductor is adjusted to provide a light emitting device capable of emitting any desired monochromatic light within a wide range of colors from purple to orange. In recent years, use is made of the characteristics of nitride semiconductors to provide practical applications of a blue light emitting diode or a green light emitting diode, and a bluish-purple light emitting semiconductor laser is being developed for use as a semiconductor laser device. When forming a nitride semiconductor film, a sapphire, SiC, spinel, Si, or GaAs substrate is used.
For example, when a sapphire substrate is used, a GaN or AlN buffer layer is formed in advance at a relatively low temperature of 500xc2x0 C. to 600xc2x0 C. prior to the epitaxial growth of a GaN film, and thereafter, the substrate is heated to a high temperature of 1000xc2x0 C. to 1100xc2x0 C. to have the GaN film epitaxially grown. It is known that, in this manner, a GaN film having good surface properties and being formed of a structurally and electrically good crystal can be produced. In addition, when an SiC substrate is used, it is known that a thin AlN film may be used as a buffer layer when the GaN film is epitaxially grown.
When a substrate other than that of a nitride semiconductor such as GaN is used, however, numerous crystal defects will be introduced in the produced nitride semiconductor film due to the differences of the thermal expansion coefficient and of the lattice constant between the non-nitride semiconductor substrate and the nitride semiconductor film grown thereon. Such defects can be categorized into two groups, i.e., edge dislocation and screw dislocation, and the total dislocation density amounts to as much as about 1xc3x97109 cmxe2x88x922 to 1xc3x971010 cmxe2x88x922. These defects are known to degrade the electric characteristics of the film by trapping carriers and also to result in the degradation of the lifetime of a laser having a large current flowing through it.
Thus, attempts are being made to form a thick film such as a GaN thick film using the hydride vapor phase epitaxy (H-VPE) method, the high pressure synthesis method, or the sublimation method and to use this thick film as a nitride semiconductor substrate in order to produce a nitride semiconductor film with reduced lattice defects and with good electric characteristics. When a nitride semiconductor thick film is used as the substrate, the thick film is doped with impurities to reduce the electric resistance that is brought high due to the reduction in the defect density and to the decrease in the residual carrier concentration in the thick film.
When the GaN thick film is to be grown by the H-VPE method as in the conventional example, however, doping of impurities of a prescribed concentration alone does not ensure good quality and electric characteristics of a crystal of the GaN thick film obtained (hereinafter, the term xe2x80x9cthick filmxe2x80x9d will be used to signify a film having a thickness of 20 xcexcm or greater unless otherwise specified). For instance, the use of a GaN thick film substrate formed by doping impurities of a prescribed high concentration for a nitride semiconductor laser device lowers the threshold voltage of the laser device but tends to increase the threshold current density. Moreover, the surface morphology of the GaN thick film substrate becomes very poor, and the quality of the crystal also is not satisfactory.
On the other hand, when a GaN thick film substrate formed by doping impurities of a prescribed low concentration is used for the nitride semiconductor laser device, the surface morphology of the GaN thick film substrate is good and the threshold current density of the laser device is lowered, but the threshold voltage tends to increase, instead. Thus, a nitride semiconductor substrate (for example, a GaN thick film substrate) that does not degrade the electric characteristics of the nitride semiconductor light emitting device is desired.
In view of the above-described background art, an object of the present invention is to provide a nitride semiconductor light emitting device having excellent characteristics using a nitride semiconductor thick film substrate.
To solve the above problems, it is required to establish a good electrical contact between the surface of a nitride semiconductor thick film to be used as the substrate and the light emitting layered structure formed by epitaxial growth. It is important to control the growth conditions appropriately so that good continuity of the epitaxial growth can be ensured.
According to the present invention, the nitride semiconductor light emitting device includes a nitride semiconductor substrate and a light emitting layered structure including a plurality of nitride semiconductor layers stacked on the substrate, wherein the nitride semiconductor substrate includes at least two layer regions including a first layer region of a high impurity concentration and a second layer region of an impurity concentration that is lower than the first layer region, and the light emitting layered structure is formed on the first layer region of the substrate.
The nitride semiconductor substrate is preferably of the n-type conductivity. Thus, the first layer region of the nitride semiconductor substrate preferably includes conductivity-type determining impurities for n-type. Silicon, germanium, oxygen, carbon, sulfur, selenium, or tellurium may be used as impurities to be included in the nitride semiconductor substrate.
The impurity concentration of the first layer region of the nitride semiconductor substrate preferably is 1xc3x971018 cmxe2x88x923 or higher. On the other hand, the nitride semiconductor substrate preferably includes a layer region having the lowest impurity concentration of 8xc3x971018 cmxe2x88x923 or lower. In addition, the first layer region of the nitride semiconductor substrate preferably has a thickness that is not greater than 10 xcexcm.
The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.